Update README.md

Author: TimGoTheCreator

README.md

@@ -38,5 +38,9 @@ python two_body.py
 ../../next two_body.txt 0.1
 ```
 
-### Next Mathematics
-NEXT uses Division-avoiding algebra optimizations to make the simulation run faster. 
+### NEXT Mathematics
+NEXT uses Division-avoiding algebra optimizations to make the simulation run faster. 
+
+### NEXT Data Output
+NEXT uses the .VTK format to output data.
+More formats like HDF5 are planned.